1. Field of the Invention
The present invention relates in general to transmission lines that are loaded with patterned permalloy or other ferromagnetic material to control domain structure orientation in the ferromagnetic material and thereby facilitate an increase in operating frequency limits for the transmission line. In addition, the patterned permalloy provides magnetic shielding. The invention also includes a novel technique for characterizing high frequency parameters of the patterned permalloy or other ferromagnetic material.
2. Description of the Background Art
Mutual inductance and self-inductance of global interconnects are crucial factors of integrated circuit performance but are difficult to extract and model in deep sub-micrometer VLSI designs. The self-inductance of signal lines helps save power by reducing signal rise times and by reducing the number of required repeaters, which in turn saves on chip area. Depending upon the circuit conditions, the signal propagation delay is either improved or not much affected when higher inductance is involved. However, mutual inductance complicates the system design dramatically through crosstalk. In fact, mutual inductance may limit the maximum usable line length when wide separations for intralayer crosstalk containment are employed.
To tackle the mutual inductance crosstalk problem, several methods have been used previously, including use of large spacing between the interconnect lines, close-by ground planes (if the targeted line impedance can still be achieved), and shorter parallel run lengths between signal lines. Unfortunately, each of these known methods share the same drawback that they deteriorate the data rate capacity of the available routing resources.
The complications are also exemplified by the difficulties in interconnect inductance extraction and modeling. Both self and mutual inductances are long-range effects and depend on the current return path, which is unknown prior to extracting and simulating the entire nonlinear circuit. As a result, no full-chip inductance extraction method is available without localization assumptions, even with the help of the partial inductance method.
It is known that magnetic materials can be used to suppress mutual inductance and provide magnetic shielding. However, for high speed VLSI applications, the magnetic materials must work in the microwave frequency range with large saturation magnetization and small coercive forces. Insulating ferrites are difficult to use because of their low saturation magnetization and CMOS process incompatibility. On the other hand, thin film ferromagnetic materials, such as permalloy, are promising for high-speed interconnect and monolithic microwave device applications due to their high permeability and CMOS compatible fabrication processes. However, ferromagnetic resonance (FMR) and eddy-current loss have limited their high-frequency applications until now. Sustained efforts in increasing the natural FMR frequency have not been very successful so far. As a result, a strong DC magnetic field, which can be very cumbersome, is usually applied for microwave applications to control FMR effects. The previous efforts to restrain FMR and eddy-current losses in as-deposited permalloy films have shown that geometry design is effective up to 6 GHz. However, this frequency is not high enough for future high-speed on-chip interconnect applications, which require operational frequencies up to 20 GHz.
The lack of accurate high-frequency characterization methods is another obstacle in developing monolithic ferromagnetic microwave devices. It has been demonstrated that up to 6 GHz, thin ferromagnetic films can be accurately characterized by the use of a signal coil technique. For broader frequency ranges, the published methods have one restriction or another. For instance, one broadband transmission line method is restricted to flexible ferromagnetic ribbons or wires, while other proposed methods require approximate theoretical expressions. Another problem with current VLSI designs is the lack of effective mutual magnetic field shielding, which limits the maximum un-buffered interconnect line length.
In view of the foregoing, a need therefore remains to develop a CMOS compatible technology that can suppress the effects of FMR and eddy currents to facilitate high frequency operation in the 20 GHz range. In addition, the technology should be able to suppress the mutual inductance, simplify the inductance extraction and modeling and increase the interconnect self-inductance. It is also desirable that the new technology provide magnetic field shielding capabilities yet preserve the data rate capacity of the given routing resources. A need also remains for a suitable high frequency characterization method for ferromagnetic microwave devices.